In microwave circuits today, traditional metallic waveguide is being replaced by alternate structures, such as the microstrip (strip line), the coplanar line, and the slot line. This is particularly true for microwave integrated circuits where metallic waveguide cannot be used. The microstrip transmission line consists of a dielectric (usually a printed circuit board) with a thin copper ground plane on one side and a copper strip (the conductor) on the other side. The coplanar line is essentially the same structure as the microstrip except that the conductor strip and the ground plane are physically on the same side of the dielectric. The conductor strip in the coplanar line obviously must be insulated from the ground by an appropriate separation. And the slot line consists of a dielectric and copper ground plane with the ground plane being interrupted by a narrow slot or gap. The slot then becomes the microwave transmission line. A discussion of slot line parameters, including methods of coupling to a slot line, is found in "Slot Line Characteristics", by E. A. Mariani, et al, IEEE Transactions on Microwave Theory and Technique, Vol. MIT-17, pp. 1091-1096, Dec. 1969.
There now exists several standard methods of coupling a microwave signal from one type of structure to a slot line structure. The standard transition used for coupling from the microstrip-to-slot line is to overlap the two microwave lines. The microstrip conductor is on one side of the dielectric and the slotted ground plane (slot line) is on the other side. In microstrip-to-slot line coupling, the two transmission lines extend approximately one-quarter wavelength beyond the transition or crossing point. These quarter-wavelength sections represent frequency sensitive stubs, and when the microstrip and slot lines cross each other at right angles, the coupling will be especially tight and relatively wideband.
Another technique is used for wideband coupling between semi-rigid or miniature coaxial line to slot transmission line. The center conductor of the coaxial line is looped across one end of an electrically open-circuit slot line and soldered to the ground plane on the other side of the slot. The shielding outer conductor of the coaxial line is also connected to the ground plane. This wideband transition utilizing coaxial line is obviously limited to particular impedances such as 50 .OMEGA. or 70-75 .OMEGA. since these are the characteristic impedances of standard microwave coaxial lines. The microstrip-to-slot line coupling is not so limited by particular impedances since impedance in the microstrip and slot line is determined by the physical dimensions and electrical characteristics of the ground plane, conductor, and dielectric.
In complex microwave designs using microstrip, coplanar, or slot line techniques, it is often advantageous to make a transition from either a microstrip or coplanar circuit to a slot line circuit close to a point in the non slot line circuit where the impedance is not the same as the slot line. Since the two circuits must have the same characteristic impedance for optimum coupling and to prevent reflections, an impedance transformation must first be performed. This can be cone by effecting the impedance transformation with a capacitor in one medium (e.g. microstrip) first to obtain a standard impedance (e.g., 50 .OMEGA.). Following this transformation, one of the other standard coupling transitions would be used. For example, to go from a microstrip having an impedance, Z.sub.o, to a slot line having an impedance, Z.sub.s, the microstrip would be coupled first to an appropriate valued series capacitor. A coaxial cable would be connected to the other side of the capacitor, and then the standard coaxial line-slot transmission performed.